Motion vectors filling area beneath?

I’m trying to use NVEncode in motion estimate mode only. I updated sample app to support pngs and I’m getting quite weird results. What am I missing?
I’m calculating motion vectors from 2 pictures of moving circle:

magick -verbose -size 1024x1024 xc:skyblue -fill red -draw "circle 300, 300,350,350" box1.png

and second image (circle moved right by 10px):

magick -verbose -size 1024x1024 xc:skyblue -fill red -draw "circle 310, 300,360,350" box2.png

and I’m getting motion vectors like this (red means non zero motion vector at that point, light green no movement):

Somehow motion vectors gets repeated vertically. I tried with video, with other images and I get this kind of picture all the time. In this example red fills until bottom, but in more complex example it fills until next movement is found.

The motion vectors from the previous row of MB’s are used as hints by NVENC, and given that the previous row has the motion, it may bias the motion towards that, given that the cost of encoding is same (the region in the bottom rows is identical even after reconstructing the MB).

This should not happen on more realistic video frames. Can you share examples of frames where it happens in real video?

Thanks for reply. After working with NVEncode for a while I suspected something like that. I found a way to keep only relevant non zero motion vectors. I use motion estimate on computer generated images/videos so there can be single color areas. Probably in real life videos that’s not the case.